Add missing JS_FreeValue calls for early returns

[sublimator]

📌 Pull Request Overview

🚩 Issue / Context

This PR addresses potential memory leaks and incorrect handling of return values in QuickJS-based transaction serialization (JS_JSONStringify calls), particularly around the correct freeing of JavaScript values and C-string conversions.

✅ What Has Changed?

• Explicit handling of JS_JSONStringify return values:

  • JS_JSONStringify can return:
    • A valid string (JSValue requiring GC cleanup).
    • An exception (JS_EXCEPTION), represented as a special tagged union value (JS_MKVAL(JS_TAG_EXCEPTION, 0)), which doesn't require cleanup.
    • JavaScript's undefined, represented similarly by a tagged value (JS_MKVAL(JS_TAG_UNDEFINED, 0)), also requiring no cleanup.
  • Added proper checks for both JS_IsException and JS_IsUndefined, handling these special tagged values gracefully and safely.

• Ensuring correct memory management:

  • Calls to JS_FreeCString and JS_FreeValue are now explicitly placed after every successful conversion to avoid leaks.
  • Added defensive checks to prevent memory leaks upon early returns (such as large string size limits).

🔍 Explanation of JS_MKVAL Values

JS_MKVAL creates lightweight, non-garbage-collected QuickJS "value types":

#define JS_MKVAL(tag, val) (JSValue){ (JSValueUnion){ .int32 = val }, tag }

• These are value types: they're copied directly on assignment (no pointers involved), meaning they require no explicit cleanup (no JS_FreeValue needed).
• Examples: JS_EXCEPTION, JS_UNDEFINED, and integer primitives.
• Confirmed by testing and valgrind analysis, there's no memory leak risk with these special values.

⚠️ Important note about JS_JSONStringify

JS_JSONStringify will explicitly return:

• JS_EXCEPTION when there's an internal error (e.g., cyclic structures or invalid replacer).
• JS_UNDEFINED when it can't serialize a given input (e.g., undefined, function, or symbol).

Both are special JS_MKVAL values (value types), hence no GC cleanup required.

🚀 Summary of PR Changes

• ✅ Bugfixes: Memory leak fixes by explicitly cleaning up returned strings and JSValues.
• ✅ Robustness: Proper handling of special QuickJS return values (exception, undefined).
• ✅ Testing: Confirmed correctness via explicit valgrind checks and careful manual reviews.

[sublimator]

Maybe want to handle this error better ....
out += "<failed-to-stringify>" or some such

[sublimator]

Maybe just use !JS_IsString ?

[sublimator]

matches existing "<could not display hex>"

 if (JS_IsBool(as_hex) && !!JS_ToBool(ctx, as_hex))
    {
        auto in = FromJSIntArrayOrHexString(ctx, data, 64 * 1024);
        if (in.has_value())
        {
            if (in->size() > 1024)
                in->resize(1024);
            out += strHex(*in);
        }
        else
            out += "<could not display hex>";
    }

[sublimator]

merged manually